This invention pertains to a flywheel uninterruptible power source and more particularly to a flywheel energy charging system that provides increased life, reliability and efficiency over previous designs. The charging system also includes a safety mechanism that prevents operation of the flywheel when the vacuum surrounding the flywheel is degraded.
Flywheel uninterruptible power supplies have emerged as an alternative to electrochemical batteries for prevention of power interruptions to critical loads. Electrochemical batteries used in these applications, in particular, valve regulated lead acid batteries, have many undesirable traits. The life of batteries is short, typically between 1 to 7 years depending on the environment and use. They require periodic maintenance and inspection, are subject to thermal degradation and can fail unpredictably. Lead acid batteries and other types as well are also environmentally noxious. However, lead acid batteries are relatively inexpensive. Flywheel systems show promise to eliminate the disadvantages of batteries with the expectation of achieving 20 year lives with minimal or no maintenance, temperature insensitivity, previously unachievable reliability while being environmentally benign.
A flywheel uninterruptible power source is shown in FIG. 1. The power source 10 includes a high-speed flywheel 12, in which energy is stored in the form of rotating inertia. Flywheels can be either constructed of metal or of composite materials. The flywheel is supported for rotation using upper and lower bearings 14 and 15. The flywheel can be supported on mechanical bearings, magnetic bearings or a combination. An attached motor/generator 16 is used to accelerate and decelerate the flywheel 12 for storing or retrieving energy. Many designs of motor/generators exist and can be employed. Motor/generators can also be made as separate components. To reduce the losses from aerodynamic drag, the interior 13 of the housing 11 surrounding the flywheel 12 is maintained at a low pressure, or for slower flywheels it can be filled with a gas of small molecule size such as helium. The flywheel uninterruptible power supply 10 includes electrical connection for operation and conversion of power. Utility power 21 is input to the input conversion 20 and power is supplied to a critical load 19 through output conversion 18. A system control 17 provides control for the system 10. The system control 17 controls the operation of the flywheel 12 by limiting currents 22, controlling speed 23 and also by monitoring parameters through diagnostics 24.
Regardless of the physical design employed, the operating life of the power source and its components is preferably maximized in order to offset the higher initial cost of the flywheel system over batteries by actually becoming cheaper when considered over the life of the power source. One element of flywheel uninterruptible power supplies that deserves particular attention is the power system electronics. Designing electronics for an operating life that is preferably greater than 10-20 years without failures is challenging.
A power system configuration of previous flywheel uninterruptible power supplies is shown in FIG. 2. The power system 30 takes in utility power 31 and supplies protected direct current power at the output 32. For many telecommunications systems such as telephone and wireless, the output voltage 32 required is xe2x88x9248 volts or 24 volts. For other applications, such as high power ride-through for data centers or critical manufacturing, the input and output voltages are higher. The input voltage 31 is rectified to a DC bus 34 using a rectifier 33 which can be either controlled or uncontrolled. The DC bus 34 supplies power to a PWM (pulse width modulated) inverter 36 also known as a servo amplifier. The servo amplifier 36 converts the DC current in the bus 34 to synchronous alternating current 37 that provides power to the flywheel motor/generator to accelerate the flywheel to normal operating speed. When the utility power is operating normally, the DC voltage in the bus 34 is converted to the output voltage 32 using a DC-DC converter 35.
During an interruption in the utility power 31, energy from the rotating flywheel supplies power to the output 32 by providing power to the DC bus. The inverter 36 provides power to the DC bus instantly and automatically when the utility power is discontinued typically by the antiparallel diodes included with the H-bridge, not shown, inside the inverter 36 or through use of a paralleled separate rectifier, not shown. Power automatically flows back and is rectified to the DC bus whenever the generator voltage is greater than the DC bus 34. As the flywheel speed slows, the voltage to the DC bus drops. The output DC-DC converter 35 maintains the constant output voltage 32 during discharging of the flywheel.
The charging of the flywheel uninterruptible power source 30 is regulated through use of the PWM inverter 36. The PWM inverter uses high frequency (xe2x88x9220 kHz) switching that chops the DC bus voltage 34 into varying width pulses that are combined to provide regulation that results in current control and speed control for the flywheel. The high frequency switching yields in several trillion cycles on the semiconductor switches in the inverter over a 20 year system life. The high number of switching cycles of the direct current stresses the inverter and could potentially result premature failures of the flywheel power source. The diagnostics of the flywheel system can also potentially have components with operating lives of concern. This is especially true if vacuum monitoring gauges are used. Vacuum gauges such as ion and thermocouple gauges are unlikely to last for the life of the power system and also are expensive
The invention is a flywheel uninterruptible power source having a charging system that provides increased life and reliability compared to previous systems. The charging system works by replacing the current regulation and/or speed control normally accomplished by a high frequency pulse width modulated inverter with a very low frequency, line commutated converter that regulates by switching the alternating current from the utility power. Pulse width modulation inverters or motor drives for high speed motors have typically 3 phase designs that invert a DC input voltage from the supply bus to a high frequency synchronous alternating current that drives the motor. In most applications, the voltage of the DC bus is fixed because it is typically the output of a fixed DC power supply.
The speed control and current control for the motor are achieved by using high frequency (xcx9c20 kHz) pulse width modulation inside the motor drive. The very high frequency is required so that the speed of the motor can be held nearly constant and free of pulsations. In contrast, the invention takes into account the very unusual application of flywheel systems. An energy storage flywheel is a unique application for a high-speed motor. The high-speed motor is coupled to a very large rotational inertia for the sole purpose of storing energy. Such flywheels can take 8 hours or more to accelerate from stopped to 30,000 rpm. Because of the large inertia coupled to the high-speed motor, speed fluxuations and pulsations are not an issue. Regardless of any torque pulsations, the only goal is to store energy in the rotating flywheel and if the energy is added in pulses, it makes no difference because the energy is being added. Therefore, the disclosed invention makes use of the uniquely stable rotational speed of energy storage flywheels by reducing the speed regulation frequency to conventional line current frequencies, frequencies under 200 Hz and 60 Hz in particular.
By reducing the switching frequency, the switching losses are reduced and the life and reliability of the switches are greatly extended. The switching losses are linearly proportional to the switching frequency. The drawback of the lower frequency switching is increased size of the inductor and capacitor filter components, however the increased life is more important for flywheel systems. The larger filter components can also be included inside the already large and heavy flywheel unit. The placement of the motor power regulation switching is moved from the motor drive preferably to the input AC line current that provides power to charge the flywheel system. The switching is preferably done using natural commutation so that the devices are turned off when the current passes through zero for very low loss and device stresses. Preferable devices for switching include thyristors or triacs. The turn on switching can be accomplished using phase angle firing or in one embodiment zero cross over switching is employed to reduce harmonic distortion and radio frequency interference to the primary source.
In another embodiment, the direct current voltage that is supplied to the inverter controlling the motor is increased by 1.5 times or more than if the primary source voltage was directly rectified. The advantage of increasing the voltage sent to the inverter is that the resistive losses in the motor/generator and other components can be reduced, the wire size and vacuum wire feedthrough sizes can be reduced and depending of the output conversion method, more energy can be extracted from the flywheel by allowing it to be discharged over a greater range of speed. The voltage can be simply increased by a use of a voltage multiplier rectifier and a version of voltage doubling rectifier can also allow for dual voltage installation. Alternatively, the voltage can be increased using a step up transformer, which has the benefit of providing isolation for the flywheel system.
In yet another embodiment, the charging system of the flywheel uninterruptible power source prevents acceleration of the flywheel if the drag is determined to be above a certain level. This is an important safety feature in preventing the flywheel from overheating. Drag can be caused by the bearing system, inadequate vacuum level or other sources. The charging system can measure the drag on the flywheel through the acceleration at a given speed and charging current. The threshold level of drag that signals to prevent accelerations can increase as the speed increases. By this method, the rotating flywheel can detect the level of vacuum and importantly can detect if the vacuum level is insufficient prior to acceleration to high speeds. This is particularly important for composite flywheels which can easily over heat and fail with a poor vacuum due to the high speed and low thermal capability and conductivity. Alternatively, the flywheel drag can be found by the level of current required to maintain a certain speed. The advantage of determining drag from the flywheel""s performance is that conventional vacuum gauges can be eliminated and the life of the system greatly increased. Vacuum gauges such as thermocouple and ion gauges are expensive and not robust. The invention is applicable for use in types of flywheel systems including long term and short term, high power and low power flywheel uninterruptible power sources.